Miniaturization of the front-end of the wireless transceiver offers many advantages including cost, the use of smaller number of components and added functionality allowing the integration of more functions. Micro-electromechanical system (MEMS) is an enabling technology for miniaturization and offers the potential to integrate on a single die the majority of the wireless transceiver components, as described by a paper by D.E. Seeger, et al., presented at the SPIE 27th Annual International Symposium on Microlithography, Mar. 3-8, 2002, Santa Clara, Calif., entitled “Fabrication Challenges for Next Generation Devices: MEMS for RF Wireless Communications”.
A micro-electromechanical system (MEMS) switch is a transceiver passive device that uses electrostatic actuation to create movement of a movable beam or membrane that provides an ohmic contact (i.e. the RF signal is allowed to pass-through) or a change in capacitance by which the flow of signal is interrupted and typically grounded.
Competing technologies for MEMS switches include p-i-n diodes and GaAs MESFET switches. These, typically, have high power consumption rates, high losses (1 dB or higher insertion losses at 2 GHz), and are non-linear devices. MEMS switches on the other hand, have demonstrated insertion loss of less than 0.5 dB, are highly linear, and have very low power consumption since they use a DC voltage and an extremely low current for electrostatic actuation. These and other characteristics are fully described in a paper by G.M. Rebeiz, and J.B. Muldavin, “RF MEMS switches and switch circuits”, published in IEEE Microwave, pp. 59-71, December 2001.
U.S. Pat. No. 6,876,282 to Deligianni et al, of common assignee, herein incorporated by reference, describes the design of a MEMS RF switch wherein the actuators being totally decoupled from the RF signal carrying electrodes in a series switch. If the actuation and RF signal electrodes are not physically separated and are part of the closing mechanism (by including one of the actuator electrodes) it may cause the switch to close (hot switching), thus limiting the switch linearity by generation of harmonics. This is a known problem for transistor switches such as NMOS or FET. Thus, in order to minimize losses and improve the MEMS switch linearity, it is important to separate entirely the RF signal electrodes from the DC actuator electrodes. U.S. Pat. No. 6,876,282 describes various designs of composite metal-insulator MEMS switches. The preferred metal used is, typically, copper, while the insulator is silicon dioxide, resulting in full separation of the actuators from the RF signal carrying electrodes. In addition, patent application Ser. No. 10/315,335 describes the use of a metal ground plane 3-4 microns below the MEMS switch to improve its insertion loss switch characteristics.
As a result of the composite metal-insulator concept, MEMS switches can be fabricated using processes that are similar to the fabrication of copper chip wiring. Integration of MEMS switch with the back-end-of-the-line CMOS process limits the material set selection and the processing conditions and temperature to temperatures no greater than 400° C.
U.S. Pat. No. 5,578,976 to Yao et al. describes a micro-electromechanical RF switch, which utilizes a metal-metal contact in rerouting the RF signal at the switch closure. MEMS metal-to-metal switches have reported problems with increases contact resistance and contact failure during repeated operation, as described by J.J. Yao et al., in the paper “Micromachined low-loss microwave switches”, J. MEMS, 8, 129-134, (1999), and in the paper “A low power/low voltage electrostatic actuator for RF MEMS applications”, Solid-State Sensor and Actuator Workshop, 246-249, (2000). Switch failure at hot switching reported to be due to contact resistance increase and contact seizure as described by P.M. Zavracky et al. in the papers “Micromechanical switches fabricated using nickel surface micromachining”, J. MEMS, 6, 3-9, (1997) and “Microswitches and microrelays with a view toward microwave applications”, Int. J. RF Microwave Comp. Aid Eng., 9, 338-347, (1999). Therein are reported an increased contact resistance and contact seizure, both of which can be associated with material transfer and arcing/welding. An Au-Au contact resistance increase to a value greater than 100 ohms was observed after two billion cycles of cold switching in N2 (no current flow through the switch), while the contact seizure was observed with hot switched samples after a few million cycles in air, as described in the aforementioned first paper.
If the switch is packaged in a hermetic environment, the contamination build up caused switch failure is less likely than when exposed to ambient conditions. When the probability of formation of a contamination film is reduced, increases in contact resistance and/or contact seizure are both due to adhesion at the metal-metal contact. The increase in contact resistance most likely has to do with material transfer caused by surface roughening and results in reduced contact area. In the latter case the two metal surfaces are firmly adhered due to metal-metal bond formation (welding) at the interface. The invention described herein is a method of fabrication of a metal-metal switch with long lifetime and with stable and low contact resistance.
Accordingly, the main thrust for reducing adhesion while gaining adequate contact resistance is:                1) different metallurgy on each side of the contact                    lattice mismatch reduces adhesion, and                        2) optimized hardness of the metals in contact                    harder metal is expected to give lower adhesion.                        
The contact metallurgy is selected not only from the group of Au, Pt, Pd as in U.S. Pat. No. 5,578,976, but also from Ni, Co, Ru, Rh, Ir, Re, Os and their alloys in such a manner that it can be integrated with copper and insulator structures. Hard contact metals have lower contact adhesion. Furthermore, hardness of a metal can be changed by alloying. Au has low reactivity, but is soft and can result in contacts that adhere strongly. For instance, to avoid this problem, gold can be alloyed. Adding about 0.5% Co to Au increases the gold hardness from about 0.8 GPa to about 2.1 GPa. Moreover, hard metals such as ruthenium and rhodium are used as switch contacts in this invention. Dual layers, such as rhodium coated with ruthenium, with increasing melting point are used to prevent contact failure during arcing where high temperatures develop locally at the contacts.